Apparatus based on shutter function for projection of a stereo or multichannel image

ABSTRACT

An apparatus for projecting a moving or still stereo or multichannel image. At least a first and at least a second video projector, or the like, project images intended for the left and the right eye of a viewer alternately in time and in synchronization with the function of shutter glasses or the like worn by the viewer. The video projectors are provided with a shutter/several shutters to prevent the projection of an image, if necessary. The control of the shutter is arranged in synchronization with the showing of the images in the following manner. When the right-eye shutter of the shutter glasses or the like worn by the viewer is open, the shutter in connection with at least the second projector projecting the image intended for the left eye is/are controlled to be shut. Correspondingly, when the left-eye shutter of the shutter glasses or the like worn by the viewer is open, the shutter in connection with at least the first projector projecting the image intended for the right eye is/are controlled to be shut. The invention enables the projection of a high-quality stereo image by means of affordable video projectors, for example liquid crystal video projectors functioning at the normal image frequency.

FIELD OF THE INVENTION

The present invention relates to an apparatus for projection of a stereoor multichannel image.

BACKGROUND OF THE INVENTION

Depth perception, or the capacity to perceive the environment in threedimensions, is based on the fact that the right and the left eye of aviewer look at the environment from a slightly different point and froma slightly different angle. Therefore, the images perceived by the rightand the left eye differ from each other to some extent, and thedifference between said images makes it possible to perceive thesurrounding space in three dimensions.

It is known from prior art to record images in such a way that the3-dimensional impression corresponding to the image recording situationis transmitted to the viewing person. These techniques are generallyreferred to as stereo imaging, and they are thus, in one way or another,based on the fact that a slightly different image is transmitted to theviewer's right and left eye.

One well-known example of the use of stereo imaging is Viewmaster™viewing devices which make it possible to look at still images recordedon a photographic film separately for the right and the left eye bymeans of a viewing device to be placed in front of the viewer's eyes.The stereo images are recorded on the film by means of a special camerausing two separate objective lenses.

Stereo imaging is used not only for presenting still images but alsomoving images.

From prior art, solutions are known for projecting a stereo image onto ascreen by means of a film projector, wherein the same image can belooked at by several viewers, for example in a cinema. In this case, theviewers wear special eyeglasses to prevent the image intended for theright eye from being visible to the left eye, and vice versa. In thecase of film projectors, the function of said glasses may be based oneither polarization or colour filtering; in other words, the imagesintended for the right and the left eyes are encloded differently bymeans of polarization or colour coding. The image to be projected on thescreen simultaneously includes superimposed images which have beenencoded differently for the right and the left eye and from which,thanks to the special eyeglasses worn by the viewer, different imagesare conveyed to the viewer's right and left eye, to produce a3-dimensional impression.

At present, when image material is primarily in electronic format andcan thus be reproduced, for example, by means of a video signal, videoprojectors are used instead of film projectors to project a stereo imageonto a screen or another projection surface. The image material to bepresented by video projectors may be a stereo image recorded by specialcameras with two objective lenses, or an image edited from an ordinaryimage by means of image processing, or also a stereo image developed ina completely “artificial” manner by means of a computer. At present, theprojection of stereo images is mostly used in the reproduction of imagematerial produced by a computer, for example to create various virtualenvironments.

A stereo image can be transferred and reproduced in electronic format bymeans of a special stereo video signal, in which the images intended forthe right and the left eye are arranged to alternate one after the otherin time. Consequently, the image frequency of the stereo video signal isdouble compared to the image frequency of a conventional video signal,if the image frequency of the images intended for the right and the lefteye is to be maintained at the normal image frequency level (25 or 30images per second).

Consequently, when said stereo video image is presented to a viewer bymeans of a wide screen television projector, the images intended for theright and the left eye are thus projected in an alternating manner oneafter the other in time. To control these successive images to beperceived in the correct manner by the viewer's right and left eye, theviewer wears so-called shutter glasses, whose operation is synchronizedfor showing images in succession. The shutter of the shutter glasses infront of the right eye is shut when the image intended for the left eyeis projected, and in a corresponding manner, the shutter in front of theleft eye is shut when the image intended for the right eye is projected.The operation of the shutter glasses is typically based on the use ofliquid crystal shutters.

When a single video projector is used for projecting a stereo videosignal, this will set considerable demands on the video projector,because the video projector must now, in principle, be capable ofoperating at a double image frequency (50 or 60 images per second)compared to the normal image frequency (25 or 30 images per second).When a computer image is being displayed, the image frequencyestablished for images intended for one eye is even higher than fornormal video images, 60 images per second, wherein the image frequencyfor stereo images becomes as high as 120 images per second.

Because of this, single video projectors which are suitable forprojecting a stereo video signal are thus based on the CRT (cathode raytube) technology, well known as such, which makes such high imagefrequencies possible but which also makes the structure of projectorsrelatively complex and thereby expensive. For example, liquid crystalvideo projectors, which are less expensive and otherwise considerablysimpler from the technical point of view, cannot be used for thispurpose in a corresponding manner, because due to the lag of liquidcrystal matrix elements they do not have the capacity for thesufficiently high projection rates required by the double imagefrequency.

For the above-mentioned reason, solutions have also been developed, inwhich image signals intended for the right and the left eye areseparated from the stereo video signal to two different videoprojectors. Thus, instead of special CRT projectors, the videoprojectors used can be less expensive projectors operating at a normalimage frequency, for example projectors based on the liquid crystalmatrix. The images of said two projectors are focused on top of eachother, superimposed simultaneously by crosswise light polarizations, andthe image thus formed is viewed through eyeglasses polarized crosswisedifferently for the right and the left eye. Such an arrangement isknown, for example, from the publication WO 00/55687.

However, such solutions of prior art, using two projectors, are limitedexpressly to the use of polarization (or colour filtering) to separatethe images intended for the right and the left eye from each other. Inother words, in these systems, the images intended for the right and theleft eye are projected to the viewer simultaneously and not sequentiallyin time as in systems using shutter glasses.

However, the use of polarization or colour filtering involves obviousdisadvantages when compared with methods based on shutter glasses. Insystems based on polarization, for example, tilting of the head by theviewer will change the filtering capacity of the polarization glassesworn by him/her. This is problematic when a stereo image is used, forexample, to create a virtual environment, in which virtual environmentthe viewer is expected to move in relation to the stereo image projectedto him/her. Both in systems using polarization and in those using colourfiltering, the natural reproduction of colours is naturally poorer thanin systems based on shutter glasses, in which the colour reproduction issubstantially neutral when the liquid crystal shutters of the shutterglasses are open. Also, both the polarization and the colour filterssignificantly reduce the brightness of the image perceived by theviewer, because the image is now typically both projected and viewedthrough optical filters with a limited transmission.

In systems based on shutter glasses and using two projectors, theproblem in practice is that the operation of the projectors should beprecisely synchronized with each other to achieve a high image quality.In other words, the projector showing the image intended for the righteye must, in principle, not project the image when the other projectoris projecting the image intended for the left eye. In spite of liquidcrystal shutter glasses, or the like, worn by the viewer, if the videoprojector producing the image intended for the left eye projects theimage or otherwise emits light when the shutter of the shutter lense infront of the right eye is open, it can also be seen by the right eye,because in this case polarization or colour filtering is thus not usedfor the encoding of images. The above-mentioned unwanted “leaking” ofthe image or light crosswise to the other eye will naturally impair thedepth impression and contrast of the stereo image as well as causeflicker of the image.

In liquid crystal video projectors, in which the light transmitted by alight source penetrates the liquid crystal matrix, and the image formedonto the liquid crystal matrix is projected by means of lense opticsfurther to be superimposed on a projection surface, the limited rate ofoperation of the liquid crystal matrix will make the above-describedsynchronization of the images more difficult. Furthermore, a perfectcontrast cannot be produced by the liquid crystal matrix, whoseoperation in the projector can be compared with the function of a slidein a slide projector. In other words, even though the aim is to controlthe liquid crystal matrix to be non-transparent, i.e. to reproduce acompletely black image, a given portion of the light from the lightsource is always passed through the liquid crystal matrix and furtheronto the screen. This unwanted light impairs the quality of the stereoimage produced.

Because of these facts, the production of a high-quality stereo image bymeans of two separate video projectors and the technique based onshutter glasses always requires, in practice, the use of videoprojectors which are of a relatively high quality and thereby relativelyexpensive. This is emphasized particularly in the reproduction of astereo image produced by means of a computer, in which the total imagefrequency of the stereo image is even higher than that of a stereo imageformed of a normal video image. Thus, in practice, the advantage to beachieved in the reproduction of a stereo image by means of twoprojectors in relation to the use of one special projector based on theCRT technology remains small, or, accordingly, the quality of the stereoimage remains significantly poorer than the image quality achieved withthe special projector.

SUMMARY OF THE INVENTION

The primary aim of the present invention is to present an apparatuswhich makes it possible to project a high-quality stereo image by usinginexpensive video projectors, such as liquid crystal video projectors,in such a way that, contrary to the prior art, the separation of theimages for the viewer's right and left eye can, however, be performed byusing technology expressly based on shutter glasses.

By means of the invention, together with the use of the shutter glasses,it is possible to achieve a clearly better image quality, for examplegreater image brightness and more natural colour reproduction, than insystems of prior art which are based on polarization or colourfiltering.

By means of the invention, it is possible to avoid, efficiently and withlow costs, the problems which would otherwise impair the quality of thestereo image and which are due, for example, to the synchronization ofthe projectors projecting the image to the right and the left eye,and/or the insufficient contrast when the shutter-glass technique isused.

The basic idea of the invention is to use a single or multiple separateshutter means, synchronized with the operation of the shutter glassesworn by the viewer, and thereby synchronized for the projection ofimages intended for the right and the left eye, in connection withprojectors used for superimposing a stereo image. If necessary, theshutter means can be used to prevent the propagation of the imageprojected by a single projector or of other light to be perceived by theviewer.

The shutter means according to the invention are efficient in preventingthe unwanted cross-“leaking” of the image between the viewer's right andleft eye, caused by the imperfect synchronization of the projectorsand/or an imperfect contrast. The shutter means according to theinvention is synchronized in such a way that when the shutter for theright eye in the shutter glasses worn by the viewer is open, the shuttermeans in connection with the projector projecting the image intended forthe left eye is controlled to be shut, and, respectively, when the leftshutter of the shutter glasses is open, the shutter in connection withthe projector intended for the right eye is shut. This arrangement willefficiently prevent the perception of an image intended for the left eyeby the viewer's right eye, and vice versa, in all situations (forexample, imperfect synchronization of projectors). Furthermore, even ifthe projector, as such, were not capable of producing a perfectcontrast, i.e., also a completely “black” image, if necessary, theshutter means of the invention will efficiently prevent the entry ofstray light, or the like, developing in that situation, on a screen oranother projection surface.

In an advantageous embodiment of the invention, the shutter means usedis a rotating disc equipped with one or more apertures and placed infront of the projectors. Such a mechanical shutter is technically easyand inexpensive to implement, and it makes 100% and 0% of lighttransmission possible when it is open and shut, respectively.

In another embodiment of the invention, the shutter means used inconnection with the projectors are shutters which are based on liquidcrystals and which can be either absorptive or dispersive by theirprinciple of operation. Liquid crystal shutters have the advantage thatno mechanical motion will be needed to implement the shutter operation,and if desired, the shutter may also be placed inside the housing of theprojector.

In yet another embodiment of the invention, the light source of thesingle projector is replaced with a light conductor which conveys lightto the projector from a separate light source placed outside saidprojector. This embodiment has the advantage that by using lightconductors, it is now possible to convey light from one light source toseveral projectors, and the shutter function according to the inventioncan be easily arranged in connection with said separate light source,for example by using a solution based on a mechanical rotating disc.

The following more detailed description of the invention by means ofexamples will more clearly illustrate, for anyone skilled in the art,advantageous embodiments of the invention as well as advantages to beachieved with the invention in relation to prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in more detail withreference to the appended drawings, in which

FIG. 1 shows, in principle, the projection of a stereo image by usingtwo video projectors according to prior art,

FIG. 2 illustrates, in principle, the use of a shutter means accordingto the invention in a situation corresponding to that of FIG. 1,

FIG. 3 illustrates, in principle, a way of using a liquid crystalshutter in connection with a video projector, to achieve the shutterfunction according to the invention,

FIG. 4 illustrates, in principle, another way of using a liquid crystalshutter in connection with a video projector, to achieve the shutterfunction according to the invention,

FIG. 5 illustrates the method of the invention applied in the projectionof a stereo image to be produced for several viewers individually, and

FIG. 6 illustrates an embodiment of the invention, in which therespective light sources of the video projectors are replaced with alight conductor and a light source outside the projectors.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows, in principle, the projection of a stereo image accordingto prior art, when two separate video projectors are used in combinationwith shutter glasses.

Image signals SR and SL intended for the right and the left eye areseparated from a stereo video signal SS, for two separate videoprojectors R and L. This can be implemented by using a decoder C,decoders being commercially available and their function beingdescribed, for example, in patent publication WO 00/55687. The rightvideo projector R and the left video projector L project an image IMonto a screen or another projection surface S in such a way that theimages intended for the right and the left eye of a viewer H areprojected one after another in time. To control these temporallysuccessive images to be perceived by the right and left eye of theviewer H in the correct manner, the viewer H wears so-called shutterglasses G, whose operation is synchronized by means of a signal SG forthe reproduction of said successive images. The signal SG is preferablygenerated in a decoder C.

The shutter L′ of the shutter glasses G in front of the left eye is shutwhen the image intended for the right eye is projected with theprojector R, and in a corresponding manner, the shutter R′ in front ofthe right eye is shut when the image intended for the left eye isprojected with the projector L. The function of the shutter glasses G iswell known, as such, from prior art and is typically based on the use ofliquid crystal shutters.

FIG. 2 illustrates, in principle, the use of a separate shutter means DSaccording to the invention in a situation corresponding to FIG. 1 toimprove the quality of the stereo image and to make it possible to usee.g. inexpensive liquid crystal video projectors as the projectors R, L.The operation of the rotating shutter disc DS used as a mechanicalshutter is synchronized by means of a signal SC, which controls therotational speed of the motor M rotating said shutter disc and ispreferably obtained from the decoder C, in the following way.

When the shutter R′ for the right eye in the shutter glasses G of theviewer H is open, the shutter disc DC prevents light emitted by theprojector L and intended for the left eye from propagating to a screen Sbut, correspondingly, allows the showing of the image projected by theprojector R, thanks to an aperture DO in the shutter disc DS. In acorresponding manner, when the left shutter L′ in the shutter glasses Gis open, the shutter disc DS prevents the showing of the image projectedby the projector R on the screen S. Consequently, the operation of theshutter disc DS is synchronized for the operation of both the projectorsR and L and the shutter glasses G.

In all situations, the above-described arrangement of the inventioneffectively prevents the perception of the image intended for the lefteye by the viewer's right eye, and vice versa. That is, for example in asituation in which the synchronization of the signals SL and SR is notquite perfect and/or the projectors R and L are, due to their principleof operation, incapable of producing a perfect contrast, or a completely“black” image, if necessary, the solution of the invention prevents theunwanted cross-“leaking” of an image or light between the viewer's rightand left eye.

The invention alleviates considerably the demands set on thesynchronization of images produced by the right R and left L projectors.When using the shutter means according to the invention, the projectorsR and L can, in principle, also be arranged to project the image in acontinuous manner, wherein the shutter means DS together with theshutter glasses G selects, from the images projected by the projectorsR, L, each time that image intended for the right or left eye which isintended to be seen by the viewer H.

With reference to FIG. 2, it is obvious that different from the figure,the aperture DO in the shutter disc DS may also have another shape thanthe circular shape. The shape of the aperture DO may, for example,correspond to a given sector portion of the disc DS. If necessary, thedisc DS may also comprise several apertures DO.

Differing from that shown in FIG. 2, it is obvious that the separatevideo signals SR and SL intended for the right R and the left Lprojectors may also be generated in another way than by decoding themseparately from the stereo video signal SS. The video signals SR, SL maybe generated directly, for example by a computer, by modifying a normalmoving or still image by various image processing means which are knownas such. The video signals SR, SL may also be generated from thebeginning by means of a computer, for example as pure computeranimations. Such a stereo image produced by a computer can be used, forexample, in computer games or for creating various virtual environmentsand spaces. From the point of view of the invention, it is thus notessential how the video signals SR and SL needed for projecting thestereo image and intended separately for the right and the left eye areproduced. From the point of view of the invention, the video signals SR,SL do not necessarily need to be combined into one stereo video signalSS at any stage. The image frequency of the video signals SR, SL mayvary according to each application, and it may thus be higher or lowerthan the image frequencies applied in normal TV application (25 or 30images per second).

It should also be mentioned that there are, at present, commerciallyavailable display adapters or the like which are to be connected to apersonal computer and which comprise several separate video outputs.Said video outputs can be programmed to produce images different fromeach other, which suitably generated images can be further used toproduce a stereo image. Such display adapters can be used for thecontrol of normal video projectors and further in combination with thesolution according to the invention to produce a high-quality stereoimage for various uses.

The invention is not limited solely to the use of the mechanical shutterDS shown in FIG. 2, although a mechanical shutter, and particularly saidshutter based on a rotating disc, is an advantageous solution as such,thanks to its simple implementation. The mechanical shutter makes 100%and 0% transmission of light possible when it is open and shut,respectively.

FIGS. 3 and 4 show some alternative embodiments of the invention inprinciple. Said figures show, in principle, the most essentialfunctional components of a so-called liquid crystal video projector LCPwhich forms an image by means of a liquid crystal matrix LC. In theliquid crystal video projector LCP, the light emitted by a light sourceF is conducted, preferably by means of a reflector P, through the liquidcrystal matrix LC and lense optics LO, to be reflected on a projectionsurface. The liquid crystal matrix LC, whose function in the projectorcan be compared with the function of a slide in a slide projector, iselectrically controlled by electronics E to produce a desired image froma video signal SR or SL input in the projector. To produce a stereoimage, two separate liquid crystal video projectors LCP are used, ofwhich one is supplied the video signal SR and the other the video signalSL, as shown in FIGS. 1 and 2.

In FIG. 3, the shutter function according to the invention isimplemented by means of a liquid crystal shutter LCS which is arrangeddirectly in front of the liquid crystal video projector in the directionof light propagation and whose operation is controlled by means of asignal SC in synchronization with the operation of the shutter glasses Gworn by the viewer. Naturally, it is obvious that in the case ofprojectors intended for the right and the left eye, said control signalSG is now generated separately for each projector.

In FIG. 4, accordingly, the shutter function according to the inventionis implemented by arranging the liquid crystal shutter LCS inside theprojector LCP, directly after the liquid crystal matrix forming theimage to be projected, in the direction of light propagation.

The liquid crystal shutters LCS to be used according to the inventionmay be of any type known as such and suitable for the purpose. Theoperation of the liquid crystal shutters LCS may thus be based on eitherlight absorption, dispersion, and/or reflection.

It is obvious that the shutter function according to the invention mayalso be arranged in other points on the light propagation path thanthose shown in FIGS. 2–4. In principle, the shutter function can bearranged at any point between the light source F of the video projectorand the projection surface S, at which point the used shutter means can,if necessary, prevent the light emitted by said projector frompropagating to the projection surface S, to be perceived by the viewerH. For example, it is possible to place the liquid crystal shutter LCSor also a suitable mechanical shutter between the light source F and theliquid crystal matrix LC.

The liquid crystal shutters LCS have the advantage that no mechanicalmotion is needed to implement the shutter function, and the shuttermeans can thus be easily placed inside the housing of the videoprojectors LCP, if necessary.

In addition to the above-described mechanical rotating disc DS andliquid crystal shutters LCS, the shutter means used can also be anyother mechanical, micromechanical or electro-optical shutter mechanismwhich is electrically controllable and has the capacity for asufficiently high operating frequency.

Moreover, the invention is not limited to the production of a stereoimage intended for one viewer only. FIG. 5 illustrates the method of theinvention applied in the projection of a stereo image to be produced forseveral viewers individually.

In FIG. 5, the video projectors A and B produce a stereo image intendedfor a viewer H from video signals SR and SL. The operation of theshutter means DA and DB is, according to the invention, synchronizedwith the operation of the shutters A′, B′ of the shutter glasses G wornby the viewer H. In a corresponding manner, video projectors C and Dproduce a stereo image intended for another viewer H′ from video signalsSR′ and SL′. In a corresponding manner, the operation of the shuttermeans DC and DD is, according to the invention, synchronized with theoperation of the shutters C′, D′ of the shutter glasses G′ worn by theviewer H′. The operation of the shutter means DA, DB, DC, DD and theshutters A′, B′, C′, D′ of the shutter glasses is synchronized with eachother and, naturally, also with the operation of the projectors A, B, C,D, as shown in the table 1 below:

TABLE 1 Synchronization of shutter means in FIG. 5. row DA DB DC DD A′B′ C′ D′ 1. open shut shut shut open shut shut shut 2. shut open shutshut shut open shut shut 3. shut shut open shut shut shut open shut 4.shut shut shut open shut shut shut open

In the application of synchronization according to Table 1, viewers Hand H′ can both be shown their own stereo images, which stereo imagemay, for example, take into account the different positions of theviewers H, H′ in relation to the projection surface S, wherein theviewers H and H′ see an object IM from different angles.

When the synchronization of the stereo image is used, for example, tocreate a virtual space, it will be possible, by using the methodaccording to the invention, to show each viewer a different high-qualitystereo image, which stereo image may, for example, represent the sameobject/view, but taking into account the viewer's different position inrelation to the image/object projected to them. Naturally, it is obviousthat the system producing the stereo image should thus know thepositions of the viewers H, H′ in relation to the projection surface S.

If necessary, the synchronization shown in Table 1 can also be changedin such a way that the table can also be scanned through, row by row, inanother order than in the order 1, 2, 3 and 4. The synchronization canbe performed, for example, in the row order 1, 3, 2, 4 or 1, 4, 2, 3. Bythis method, it is possible to reduce the flicker of the image perceivedby a single viewer H or H′.

Finally, FIG. 6 shows, in principle, an embodiment of the invention inwhich the respective internal light sources of the video projectors R, Lare replaced with light conductors OF, which light conductors arearranged to conduct light to the projectors from a separate light sourceF placed outside said projectors. The light source F may be, forexample, a continuously operating halogen or gas discharge lamp toproduce good luminosity. In connection with the light source F, aninexpensive reflector P and/or other optics is used (not shown in FIG.6) to collect and focus the light from the light source F with a goodefficiency to the light conductor/conductors OF.

This embodiment has the advantage that by using light conductors OF, itis possible to convey light from one light source F to severalprojectors, and the shutter function according to the invention can beeasily arranged in connection with said separate light source F, forexample by using a solution based on a mechanical rotating disc.

As the light conductors OF, it is possible to use quartz or plasticbased optic fibres or fibre bundles formed of them. Quartz based opticfibres have the advantage of good thermal stability which is significantwhen light with a high luminance is to be coupled to the end of thefibre bundle. It is also possible to use liquid filled light conductors.

For a person skilled in the art, it will be obvious that the coupling oflight from a light source F to the light conductors OF can be providedwith a variety of ways implementing the shutter function of theinvention. Instead of the embodiment of FIG. 6, the guidance of lightbetween different light conductors OF can be implemented, for example,by using a rotating or oscillating mirror (mirrors). To implement theactual shutter function, it is also possible, in addition to variousmechanical solutions, to use for example liquid crystal shutters or thelike.

By using light conductors OF, it is also possible to supply light tomore than two video projectors by using one continuously operated lightsource F. Using light conductors, it is thus possible to implement, forexample, the system intended for several viewers H, H′, as shown in FIG.5 in principle. Thus, a significant advantage achieved by using lightconductors is that the structure of single projectors becomes simpler,wherein the overall apparatus becomes simpler and less expensive toimplement. Leaving out individual light conductors from singleprojectors makes it also possible to implement projectors with a verycompact size.

It is obvious that the above-described different embodiments of theinvention can be combined to produce various embodiments of theinvention which comply, as such, with the spirit of the invention.Therefore, the above-presented examples must not be interpreted asrestrictive to the invention, but the embodiments of the invention canbe freely varied within the scope of the inventive features presented inthe claims hereinbelow.

Naturally, it will be obvious that even though the viewer H, H′ in theabove-presented examples is placed on the other side of the projectionsurface S than the projectors used for projecting a stereo ormulti-channel image, it is conventional, in practice, that both theseprojector apparatuses and the viewers are placed on the same side inrelation to the projection surface S. Instead of a conventional screen,the image can also be projected on other stationary or non-stationarysurfaces which are suitable for the purpose.

The synchronization of shutter glasses G worn by the viewer can beadvantageously implemented by using wireless communication means, forexample radio frequency and infrared links, which are known as such fromprior art, wherein the movement of the viewer is not limited by wiringetc. to be otherwise connected to the shutter glasses G.

Naturally, it is obvious that also the mechanical shutter disc DS or thelike can be placed inside a single video projector, if necessary. Inthis case, however, one loses the advantage of avoiding the need tomodify the projectors themselves by placing the shutter disc DS or thelike in front of the projectors.

The shutter function according to the invention can also be applied inconnection with other video projectors than those mentioned in theexamples and based on the liquid crystal matrix. In view of theinvention, another advantage of liquid crystal matrix video projectors,in addition to their inexpensive price, is that contrary to matriceswhich are refreshed and controlled row by row, the image formed by theliquid crystal matrix involves no significant scanning to update theimage and recur in time. When such a phenomenon occurs, it should betaken into account in the arrangement of the scheduling of the shutteraccording to the invention.

In a summary, it can be stated that the invention makes it possible toproject a high-quality stereo or multi-channel image by using two ormore separate video projectors, which operate at the normal imagefrequency and are inexpensive as such, and shutter glasses. To provide acorresponding image quality according to the prior art requires, inpractice, the use of a special stereo image projector which is based onthe CRT technology and is significantly more expensive.

1. An apparatus for projecting a moving or still stereo or multichannelimage, comprising: at least a first and at least a second videoprojector for projecting electronic image material intended for the leftand the right eye of the viewer alternately in time by means of a videosignal and in synchronization with the function of shutter glasses wornby the viewer, wherein said video projectors are provided with a shuttermeans/several shutter means to prevent the projection of the image, ifnecessary, the control of said shutter means being arranged insynchronization with the showing of the images in such a manner thatwhen the right-eye shutter of the shutter glasses worn by the viewer isopen, the shutter means in connection with at least the second projectorprojecting the image intended for the left eye is/are shut, andcorrespondingly, when the left-eye shutter of the shutter glasses wornby the viewer is open, the shutter means in connection with at least thefirst projector projecting the image intended for the right eye is/areshut.
 2. The apparatus according to claim 1, wherein said shutter meansis/are arranged outside the video projectors on the travel path of lightin the area between said projectors and the projection surface used forshowing the image.
 3. The apparatus according to claim 1, wherein saidshutter means is/are arranged outside the video projectors on the travelpath of light immediately in front of said video projectors.
 4. Theapparatus according to claim 1, wherein said shutter means is/arearranged in connection with the video projectors, inside the housing ofsaid video projectors on the travel path of the light emitted by aninner light source of the projector.
 5. The apparatus according to claim1, wherein an inner light source of the video projector is a lightconductor, which light conductor is arranged to conduct light of anexternal light source to the video projector to project an image, andsaid shutter means is/are arranged in connection with said lightconductor.
 6. The apparatus according to claim 1, wherein said shuttermeans is/are means effecting the shutter function by means of amechanical movement.
 7. The apparatus according to claim 6, wherein anopaque disc that is rotated in a syncronized manner is used as a shuttermember, said disc containing one or more light-permeable apertures. 8.The apparatus according to claim 6, wherein the apparatus is arranged toshow two or several viewers a stereo image in such a manner that eachviewer is shown a different image.
 9. The apparatus according to claim6, wherein the video projectors used in the apparatus are liquid crystalprojectors based on the use of a liquid crystal matrix.
 10. Theapparatus according to claim 1, wherein said shutter means is/areelectro-optical shutter means.
 11. The apparatus according to claim 10,wherein said shutter means is/are liquid crystal shutters based on theabsorption, dispersion or reflection of light.
 12. The apparatusaccording to claim 10, wherein the apparatus is arranged to show two orseveral viewers a stereo image in such a manner that each viewer isshown a different image.
 13. The apparatus according to claim 10,wherein the video projectors used in the apparatus are liquid crystalprojectors based on the use of a liquid crystal matrix.
 14. Theapparatus according to claim 1, wherein the apparatus is arranged toshow two or several viewers a stereo image in such a manner that eachviewer is shown a different image, if necessary.
 15. The apparatusaccording to claim 1, wherein the video projectors used in the apparatusare liquid crystal projectors based on the use of a liquid crystalmatrix.
 16. An apparatus for projecting a moving or still stereo ormultichannel image, comprising: at least a first and at least a secondvideo projector for projecting images intended for the left and theright eye of the viewer alternately in time and in synchronization withthe function of shutter glasses worn by the viewer, wherein said videoprojectors are provided with a shutter means/several shutter means toprevent the projection of the image, if necessary, the control of saidshutter means being arranged in synchronization with the showing of theimages in such a manner that when the right-eye shutter of the shutterglasses worn by the viewer is open, the shutter means in connection withat least the second projector projecting the image intended for the lefteye is/are shut, and correspondingly, when the left-eye shutter of theshutter glasses worn by the viewer is open, the shutter means inconnection with at least the first projector projecting the imageintended for the right eye is/are shut wherein an inner light source ofthe video projector is a light conductor, which light conductor isarranged to conduct light of an external light source to the videoprojector to project an image, and said shutter means is/are arranged inconnection with said light conductor.